Frequency Downshift Research Articles

Hydrogen bonds and O⋯O interactions in proton-ordered ices. DFT computations with periodic boundary conditions

Frequencies and IR intensities of the OH stretching vibrations in proton-ordered ices VIII, II, IX and XI are computed using B3LYP/6-311G ∗∗ with periodic boundary conditions. In accord with the experiment, the average downshift of the OH or OD frequencies has a trend in changing from small to large shift in the order: VIII < II < IX < XI. The H-bond enthalpy values, evaluated from the IR intensities of the OH stretching vibrations, are consistent with this trend. PBE/6-311G ∗∗ and BLYP/6-311G ∗∗ overestimate the OD frequency downshift. The bond paths in the periodical electron density, which are associated with the O⋯O interactions in ices VIII, II and IX, have been found.

Zero‐offset profiling using frequency cross‐hole radar in a layered embankment test site: antenna design, simulation and experimental results

ABSTRACTRoad surface quality is very sensitive to the mechanical properties of an underground structure and particularly to its water content. Depth sounding using boreholes is generally used for geotechnical testing and particularly oven dried measurements. In parallel, electromagnetic measurements based on a cross‐hole radar to perform zero‐offset profiling are proposed in this paper. Thus, two specific large band dipole geometries, a 3D folded dipole and a planar blade dipole have been designed to work in the frequency band [0.5;1.5] GHz in order to estimate the vertical water content of a multi‐layered clay sand embankment test site. The design of both antenna geometries and their integration in a transmission link considering a cross‐hole radar with a separation distance less than 1 m, has been performed using the finite difference time‐domain (FDTD) commercial software EMPIRE®. The picking of the time arrivals has allowed to estimate the real permittivity and the volumetric moisture content using Malicki’s relation. The comparison of measurement and simulation results has allowed to highlight the contrasts of real permittivities induced by the volumetric water content and the embankment compaction level. Moreover, in this study the dissipative properties in the soil have been evaluated as they represent complementary depth properties closely linked to the accuracy of the first arrivals picking used for the permittivity estimation. The intrinsic attenuation evaluation is based on a straight analysis in the frequency domain of successive transmission coefficients using the centroid frequency downshift method. Afterwards the electrical conductivity profile has been recovered using a full‐wave FDTD analysis and a comparison between measurement and simulation data.

Raman scattering determination of the energy difference between Γ and L conduction band minima in Ga1−xInxAsySb1−y

We report a Raman scattering determination of the energy difference EΓ−L between the Γ conduction-band minimum and the L valley minima in n-type Ga1−xInxAsySb1−y lattice matched to GaSb (x=0.15, y=0.13). A frequency downshift in the L+ phonon–plasmon coupled mode is observed between 80 K and room temperature that is attributed to electron transfer from the Γ to the L valleys. We use the L+ frequency shift to evaluate EΓ−L by performing Lindhard–Mermin L+ line-shape fits for different EΓ−L values. The EΓ−L value increases with electron concentration due to band gap renormalization. A value EΓ−L=154 meV is derived for intrinsic material.

Laboratory observations of wave evolution, modulation and blocking due to spatially varying opposing currents

The nonlinear evolution of waves propagating on a spatially varying opposing current has been observed in a wave–current flume. Regular waves with different initial periods and different initial steepness, s (0.05 &lt; s &lt; 0.19), were generated and observed. Frequency downshift, even with very small initial steepness, was identified. As expected, it was found that opposing currents can have significant interactions with wavetrains. The ultimate frequency downshift increases with the increase in initial steepness. The evolution of frequency modulation was observed via the instantaneous frequency extracted by the Morlet-wavelet transform. The instantaneous frequency showed that often the process of frequency downshift can be local in time and gradual, but abrupt changes of local frequency were also detected. The presence of an opposing current can gradually block the primary wave energy and destroy the conservation of the wave action at downwave locations, thus increasing the asymmetric modulation and accelerating the effective frequency downshift.

Experimental evidence of stable wave patterns on deep water

Recent predictions from competing theoretical models have disagreed about the stability/instability of bi-periodic patterns of surface waves on deep water. We present laboratory experiments to address this controversy. Growth rates of modulational perturbations are compared to predictions from: (i) inviscid coupled nonlinear Schrödinger (NLS) equations, according to which the patterns are unstable and (ii) dissipative coupled NLS equations, according to which they are linearly stable. For bi-periodic wave patterns of small amplitude and nearly permanent form, we find that the dissipative model predicts the experimental observations more accurately. Hence, our experiments support the claim that these bi-periodic wave patterns are linearly stable in the presence of damping. For bi-periodic wave patterns of large enough amplitude or subject to large enough perturbations, both models fail to predict accurately the observed behaviour, which includes frequency downshifting.

Energetic-particle modes driven by suprathermal electrons produced by off-axis second harmonic ECRH in compact helical system (CHS)

Recurrent bursting energetic-particle modes (EPMs) are observed in an l = 2 helical plasma heated by second harmonic electron cyclotron resonance heating (ECRH) without neutral beam injection. The observed instabilities are characterized by a rapid frequency downshift, propagating in the ion-diamagnetic drift direction. Such bursting MHD instabilities are seen only in fairly low-density plasmas with off-axis second harmonic ECRH. A soft x-ray detector operating in the counting mode indicates that there exist suprathermal electrons in low-density ECRH plasmas where EPMs are destabilized. The excitation of EPMs is supposed to be due to the helically trapped suprathermal electrons produced by the second harmonic ECRH.

Structure and vibrational spectra of sulfur hexafluoride encapsulated α-cyclodextrin

Density functional calculations have been utilized to understand the interactions of sulfur hexafluoride (SF 6) with α-cyclodextrin (α-CD) host. An encapsulation of SF 6 (guest) within the host cavity and its binding to primary as well as secondary hydroxyl rims of α-CD exhibiting different hydrogen bonding patterns have been analyzed. The present calculations have shown that interactions between the guest and protons within the host cavity yield the minimum energy complex wherein guest penetrates from secondary hydroxyl end of the host. Calculated vibrational spectra reveal frequency downshift of S–F stretching in the guest molecule on complexation. Frequency shift in opposite direction has been noticed for O–H vibrations of secondary hydroxyls of the host those facilitate relatively strong hydrogen bonded interactions. These inferences have been supported by mapping difference molecular electron density on the bond critical points in molecular electron density topography. The electronic structure and vibrational spectra derived from the density functional calculations based on the B3LYP and the X3LYP functional are similar. Use of X3LYP functional however, predicts shortening of O6H⋯O6′ as well as CH⋯F hydrogen bonds and thus points to the strengthened host–guest interactions and in turn binding energies of SF 6 within these complexes employing the X3LYP functional turn out to be nearly ∼35–40% higher compared to those obtained within the framework of B3LYP theory.

The Benjamin–Feir instability and propagation of swell across the Pacific

About 40 years ago, Snodgrass and other oceanographers (1966) tracked ocean swell propagating across the entire Pacific Ocean. At about the same time, several investigators (including Benjamin and Feir) showed that a uniform train of plane waves of finite amplitude on deep water is unstable. Comparing these two results, each of which is highly cited, leads to the following question: in light of this instability, how did the waves tracked by oceanographers travel coherently more than 10,000km across the Pacific Ocean? A possible explanation is provided in recent work that re-examined the Benjamin–Feir instability in the presence of linear damping. The conclusion was that even small amounts of damping can stabilize the instability before nonlinear effects become important. In addition, the theoretical predictions agree well with results from laboratory experiments. In this paper we re-examine ocean data from 1966 to estimate whether the oceanic damping that was measured could have controlled the Benjamin–Feir instability for the swell that was tracked. We find that for one set of ocean swell, dissipation controls the Benjamin–Feir instability enough to allow coherent wave propagation across the Pacific. For a second set of ocean swell, it does not. For a third set of ocean swell, an integral that the theory predicts to be constant is not constant in the data; it decreases and correspondingly the spectral peak shifts to a lower frequency—this is frequency downshifting. For this case the theory is not an adequate model, so the corresponding Benjamin–Feir analysis can be misleading. Thus, our results from the historical records are inconclusive: we can assert neither that dissipation of ocean swell is always negligible, nor that it is always important. But our results show that dissipation can control the Benjamin–Feir instability for small-amplitude waves and that downshifting occurs in ocean swell with relatively small wave slopes.

Evolution of the air cavity during a depressurized wave impact. II. The dynamic field

The present paper on wave-impact events in depressurized environments completes the analysis of Part I by focusing on the dynamical features of the impacts and on the influence of the ambient pressure. Connection is made between the impact regimes typically described in the literature and the stages described in Part I [C. Lugni, M. Miozzi, M. Brocchini, and O. M. Faltinsen, “Evolution of the air cavity during a depressurized wave impact. I. The kinematic flow field,” Phys. Fluids 22, 056101 (2010)]. The stages of isotropic/anisotropic compression and expansion of the air cavity are of particular interest. The impact duration at the wall is almost independent of its height above the undisturbed surface level, but its intensity rapidly decreases in the body of the fluid (the peak pressure halves within the first two compression/expansion cycles). The time evolution of the pressure loads on the wall is analyzed by means of the Hilbert transform and an empirical mode decomposition of the signals. This enables identification of the intrinsic mode functions which best fit the original signal during its evolution and quantification of the frequency downshifting which characterize the whole process. The pressure decay, largely governed by air leakage out of the cavity, is found to be very intense during the air cavity closure and the isotropic compression/expansion cycle [stages (1) and (2)]; the decay observed during stage (3), i.e., during the anisotropic compression/expansion cycles, is weaker and independent of the vertical location down the wall. Differences between the observed decay rates and those of a three-dimensional bubble in an infinite fluid are mainly due to the bubble being two-dimensional, being close to the free surface and loosing air. The role of both ullage and vapor pressures on the impact is described, respectively, by means of the Euler and cavitation numbers. The frequency of the bubble oscillation depends on these numbers in a way that is closely similar to that displayed by the bubble area, see Part I.

Soliton self-frequency shift: Self-similar solutions and their stability

Ultrashort pulse propagation in fibers is affected by intrapulse Raman scattering (IRS) which causes both a linear frequency downshift and a quadratic displacement of the peak pulse, as functions of the propagation distance. This effect has been known and treated by perturbation methods applied to the nonlinear Schrödinger equation since the period of intense research on soliton propagation. Here, we find solutions of the model equation using an accelerating self-similarity variable and study their stability. These solutions have Airy function asymptotics which give them infinite energy. For small IRS, the algebraically decaying tail is negligible and these solutions are a very good approximation of the profiles observed in the full equation simulations. For strong IRS (but beyond the regime in which the evolution equation is valid for silica fibers), the self-similar pulses have noticeable left tails exhibiting Airy oscillations. Whenever their truncated forms are used as initial conditions of the full equation, they experience amplitude decay and show left tails that are consistent with radiation escaping from the central pulse. These observations are interpreted as being the effects of a continuum constitution of the infinite left tail.

Stress and its effect on optical properties of AlN nanorods

The stress states in AlN nanorods deposited on Si and its effect on optical properties have been investigated by means of Raman scattering and photoluminescence methods. The observed frequency downshift and linewidth broadening from temperature-dependent Raman scattering can be well described by an empirical relationship taking into account the contributions of the thermal expansion and decay of optical phonons. The phonon-energy difference of the E2(high) mode between the stress-free bulk-AlN and AlN nanorods appears to increase with increasing temperature, demonstrating that differential thermal expansion between the Si-substrate and AlN nanorods is the key parameter reflecting the stress in the nanorods.

Qualitative comparisons of experimental results on deterministic freak wave generation based on modulational instability

A number of qualitative comparisons of experimental results on unidirectional freak wave generation in a hydrodynamic laboratory are presented in this paper. A nonlinear dispersive type of wave equation, the nonlinear Schrödinger equation, is chosen as the theoretical model. A family of exact solutions of this equation the so-called Soliton on Finite Background describing modulational instability phenomenon is implemented in the experiments. It is observed that all experimental results show an amplitude increase according to the phenomenon. Both the carrier wave frequency and the modulation period are preserved during the wave propagation. As predicted by the theoretical model, a phase singularity is also observed in the experiments. Due to frequency downshift phenomenon, the experimental signal and spectrum lose their symmetric property. Another qualitative comparison indicates that the Wessel curves for the experimental results are the perturbed version of the theoretical ones.

High resistive ferrite films by a solution process for electromagnetic compatible (EMC) devices

Mn–Zn ferrite films with high resistivity and good noise suppressing ability for use as ‘coupling-type noise suppressors’ have been prepared by the spin-spray ferrite plating. The as-prepared films were crystalline and exhibited single-phase spinel structure. The films had an ‘integrated nano-columnar’ morphology that resulted in a very high resistivity. Further, by varying the chemical composition, films with varying resistivity were prepared and then heat treated at 260 °C for 3 min, similar to that of the reflow soldering process. The reflection and transmission coefficients, S 11 and S 21 parameters, on coplanar micro-strip line (50 Ω) were measured for the as-prepared and heat-treated films in order to study the effect of heat treatment. When the resistivity was above 2×10 5 Ω/sq, S 11 and S 21 exhibited uniform profiles throughout the measurement frequency (50 MHz–10 GHz), which is ideal for the ‘coupling-type’ noise suppressor. These films retain a moderately high resistivity and hence do not show the downshift in the stopband frequency even after the heat treatment (reflow soldering process).

Microstructural aspects for defect emission and E2high phonon mode of ZnO thin films

The ZnO thin films were prepared using a cost-effective vacuum-carbon-arc technique combined with thermal oxidation under ambient conditions from zinc nanoparticles in carbon matrix. The microstructural analysis of grazing incidence x-ray diffraction pattern using Rietveld method was investigated systematically under the influence of different annealing temperature. Photoluminescence spectroscopic properties of the samples studied at room temperature exhibits the dependence of violet emission accompanying with blue and green emission on oxidation temperature. The origin of defect emission mechanism through the microstructure of ZnO thin films was analyzed. The anharmonic properties of the upper E2 phonon mode were investigated using Raman scattering spectroscopy at room temperature. The downshift of phonon frequency and linewidth broadening of high frequency E2 phonon mode were analyzed through defect state according to annealed conditions.

Phase transition and phonon dynamics in NiPdSi: An annealing study by temperature-dependent Raman spectroscopy

We have demonstrated the effective Raman spectroscopy for the phase transition and phonon dynamics of NiPdSi films annealed at various temperatures. In addition to the appearance of alloy disorder-induced Raman peaks, we have shown that the presence of Pd in NiSi results not only in the retardation of NiSi2 phase transition to about 900 °C, but also in the redshift (∼3–4 cm−1) of the first-order Raman peaks. The observed Raman frequency downshift with increasing temperature can be described well by a model which has taken into account the contributions of the anharmonic, thermal expansion, and strain effects, revealing the dominant role of the phonon decay. Effects of Pd addition and annealing temperature on the Raman frequency and linewidth have been discussed. The introduction of Pd is found to reduce the Raman frequency downshift, demonstrating the favorable use of NiPdSi for microelectronics applications.

Pump depletion limited evolution of the relativistic plasma wave-front in a forced laser-wakefield accelerator

In a forced laser-wakefield accelerator experiment (Malka et al 2002 Science 298 1596) where the length of the pump laser pulse is a few plasma periods long, the leading edge of the laser pulse undergoes frequency downshifting and head erosion as the laser energy is transferred to the wake. Therefore, after some propagation distance, the group velocity of the leading edge of the pump pulse—and thus of the driven electron plasma wave—will slow down. This can have implications for the dephasing length of the accelerated electrons and therefore needs to be understood experimentally. We have carried out an experimental investigation where we have measured the velocity vf of the ‘wave-front’ of the plasma wave driven by a nominally 50 fs (full width half maximum), intense (a0 ≃ 1), 0.815 µm laser pulse. To determine the speed of the wave front, time- and space-resolved refractometry, interferometry and Thomson scattering were used. Although a laser pulse propagating through a relatively low-density plasma (ne = 1.3 × 1019 cm−3) showed no measurable changes in vf over 1.3 mm (and no accelerated electrons), a high-density plasma (ne = 5 × 1019 cm−3) generated accelerated electrons and showed a continuous change in vf as the laser pulse propagated through the plasma. Possible causes and consequences of the observed vf evolution are discussed.

A RECORDING-BASED APPROACH FOR IDENTIFYING SEISMIC SITE LIQUEFACTION AND NONLINEARITY VIA HHT DATA ANALYSIS

This study proposes an approach to identify earthquake-induced site liquefaction and/or nonlinearity from Hilbert–Huang–Transformation- or HHT-based data analysis of seismic motion recordings. The proposed approach fully utilizes unique features of the HHT method in characterizing instantaneous frequency and damping as well as temporal-frequency motion from the recordings, so as to single out and quantify liquefaction- and/or nonlinear-soil-related nonlinear phenomena shown in the recordings. With post-earthquake, site investigation as a reference, this study shows that the proposed approach is effective in characterizing site nonlinearity, quantifying nonlinear influences in site amplification, and diagnosing site liquefaction. Major results from this study are listed below.1. Predominant instantaneous frequency of earthquake motion is defined as the frequency with the largest amplitude in Hilbert amplitude spectra of the motion at a time instant, and subsequently used together with other motion features for identifying site liquefaction conditions. Analysis of 29 sets of seismic recordings with different liquefaction conditions shows that the proposed approach is more effective in detecting site liquefaction than other Fourier-based methods.2. HHT-based site amplification is defined as the ratio of marginal Hilbert amplitude spectra, similar to the Fourier-based one that is the ratio of Fourier amplitude spectra. The HHT-based site amplification can be used for effectively quantifying site nonlinearity in terms of frequency downshift in the low-frequency range and amplification-reduction factor in intermediate-frequency range in comparison with the Fourier-based one.3. Instantaneous damping, and Hilbert and marginal Hilbert damping spectra are defined in way similar to instantaneous frequency, and Hilbert and marginal Hilbert amplitude spectra, respectively. Consequently, the HHT-based site damping is found as the difference of marginal Hilbert damping spectra, which can be used as an alternative, complementary index to measure influences of site nonlinearity in seismic ground responses.

Wireless UV sensor based on photocapacitive effect in GaN

The LC oscillator with the interdigitated aluminium-on-GaN/sapphire photo capacitor has been demonstrated. Upon illumination with a 375 nm UV LED, an oscillator frequency downshift owing to the photo capacitive effect attains 5.2 MHz (3.5 of the dark frequency value 149 MHz) with sensitivity up to 170 kHz/( muW/cm 2 ). The oscillator can be employed as a visible-blind UV sensor with a remote wireless pickup of the RF output signal.

Ultrasound attenuation estimation using backscattered echoes from multiple sources

The objective of this study was to devise an algorithm that can accurately estimate the attenuation along the propagation path (i.e., the total attenuation) from backscattered echoes. It was shown that the downshift in the center frequency of the backscattered ultrasound echoes compared to echoes obtained in a water bath was calculated to have the form Deltaf=mf(o)+b after normalizing with respect to the source bandwidth where m depends on the correlation length, b depends on the total attenuation, and f(o) is the center frequency of the source as measured from a reference echo. Therefore, the total attenuation can be determined independent of the scatterer correlation length by measuring the downshift in center frequency from multiple sources (i.e., different f(o)) and fitting a line to the measured shifts versus f(o). The intercept of the line gives the total attenuation along the propagation path. The calculations were verified using computer simulations of five spherically focused sources with 50% bandwidths and center frequencies of 6, 8, 10, 12, and 14 MHz. The simulated tissue had Gaussian scattering structures with effective radii of 25 mum placed at a density of 250 mm(3). The attenuation of the tissue was varied from 0.1 to 0.9 dB / cm-MHz. The error in the attenuation along the propagation path ranged from -3.5+/-14.7% for a tissue attenuation of 0.1 dB / cm-MHz to -7.0+/-3.1% for a tissue attenuation of 0.9 dB / cm-MHz demonstrating that the attenuation along the propagation path could be accurately determined using backscattered echoes from multiple sources using the derived algorithm.

Thermal properties of guest-free Si136 and Ge136 clathrates: A first-principles study

We have used the generalized gradient approximation (GGA) to density functional theory to study the vibrational and thermal properties of guest-free Si136 and Ge136 clathrates. In order to study the effects of supercell size on our results, we have performed both 34 and 136 atom supercell calculations for each material. We find that the 34 atom supercell calculations predict a small frequency downshift (in comparison with the 136 atom supercell calculations) in the vibrational density of states of both materials. The GGA-predicted Γ phonon frequency of Si136 (480 cm−1 at T=0 K) obtained from the 136 atom calculations is in very good agreement with the experimental value for Na1Si136 (484 cm−1 at T=300 K). Using the results from our 136 atom calculations, we have also calculated the temperature dependence of the vibrational contributions to the Helmholtz free energy, the entropy, and the specific heat (CV) of the guest-free Si136 and Ge136 clathrates. The predicted and experimental heat capacities of Si136 are found to be in close agreement.